Monolith-holding element, process for producing the same, catalytic converter using a monolith member and process for producing the same

ABSTRACT

A monolith-holding element adapted to be used in an exhaust system of internal combustion engines, a process for producing the monolith-holding element, a catalytic converter and a process for producing the catalytic converter are disclosed, in which the specific monolith-holding element is produced by uniformly dispersing an organic binder in a compressed alumina fiber mat, exhibits a thickness-restoring property when the organic binder is thermally decomposed by the contact with a high-temperature exhaust gases, and supports a monolith by exerting a surface pressure on an outer peripheral surface of the monolith and an inner peripheral surface of the metal casing. The monolith-holding element has an excellent durability and gas-sealing properties and the catalytic converter can withstand severe vibration and impact for a long period of time.

TECHNICAL FIELD

The present invention relates to a monolith-holding element, a processfor the production of the monolith-holding element, a catalyticconverter using a monolith member composed of a monolith andmonolith-holding element, and a process for the production of thecatalytic converter. The monolith-holding member according to thepresent invention is maintained in a compressed state by a bonding forceof an organic binder upon assembling, and restore its inherent thicknessand exhibits a required surface pressure in a both surface-supportingstate by the thermal decomposition of the organic binder after heating.In the catalytic converter according to the present invention, theafore-mentioned monolith-holding element stably supports the monolithwithin a casing for a long period of time.

BACKGROUND ART

In exhaust systems of internal combustion engines, various catalyticconverters containing heavy metals or noble metals as a catalyst havebeen used as an apparatus for cleaning exhaust gases in order to treatand remove harmful components in the exhaust gases, such as carbonmonoxide or various hydrocarbons. These catalytic converters areclassified into the following two types according to the configurationof catalysts used therein:

(1) Catalytic converter comprising a pellet-like catalyst prepared bysupporting a metal catalyst on a granular carrier made of ceramics orthe like, and a metal casing for accommodating the pellet-like catalyst;and

(2) Catalyst converter comprising a so-called integrated catalystprepared by supporting a metal catalyst on a tubular monolith carrier(hereinafter referred to merely as "monolith") inside of which aplurality of flow paths for passing exhaust gases are provided, and ametal casing accommodating the integrated catalyst and connected withexhaust pipes.

The catalytic converters (2) have been more predominately utilized ascompared with those of the catalytic converters (1) because thecatalytic converters (2) do not show abrasion due to collision betweenpellets as observed in the catalytic converters (1) and have relativelyminiaturized the apparatus. In the catalytic converters (2), in order tosecurely mount a monolith member within a metal casing, amonolith-holding element is wound around a monolith.

In many cases, the monoliths made of ceramics or metals have been usedin order to impart a heat resistance thereto. Further, in order toreduce a flow resistance during passing of exhaust gases and enhance anefficiency of the catalyst, there have been predominately utilizedmonoliths having a honeycomb structure, whereby more larger surface areacan be assured in flow paths of exhaust gases.

As an appropriate structure of the metal casings, there has been adopteda two-piece clamshell structure in which the monolith member issandwiched between shell halves of the metal casing and the matingportions of the shell halves are welded together, or a stuffing boxstructure in which the monolith member is inserted into the metalcasing.

In either the clamshell or stuffing box structure, it is required thatthe thickness of the monolith-holding element is identical with orslightly larger than a clearance between an outer peripheral surface ofthe monolith and an inner peripheral surface of the metal casing inorder to securely fix the monolith within the metal casing. This isbecause the decrease in retaining force of the monolith-holding elementagainst the monolith causes inconveniences such as separation ordisplacement of the monolith within the metal casing, leakage of exhaustgases from the outer peripheral surface of the monolith, or the like,during operation. Occurrence of such inconveniences is highlydetrimental to the catalytic converters.

Especially, in the case where a ceramic monolith is used in thecatalytic converter, the clearance between the outer peripheral surfaceof the monolith and the inner peripheral surface of the metal casingbecomes large during operation because the monolith itself exhibits asmall thermal expansion while the metal casing exhibits a large thermalexpansion. As a result, there has been a tendency that the retainingforce of the monolith-holding element becomes decreased. In addition, inthe production of the catalytic converter, in the case where such amonolith having an extremely size and showing a relatively largedispersion in its outer dimension is used, a retaining force requiredfor securely holding the monolith cannot be often obtained uponassembling.

Accordingly, in order to obtain a desired retaining force of themonolith-holding element over a wide temperature range from a normaltemperature to a high temperature during operation, it is required thatthe monolith-holding element can follow the difference in size of theclearance caused by fluctuated outer dimensions of the individualmonoliths, or the change in the clearance due to difference in thermalexpansion between the monolith and the metal casing.

In recent years, intense studies have been made on a method for fixingthe monolith within the metal casing. As a result, as monolith-holdingelements adapted to be fitted into a clearance between the outerperipheral surface of the monolith and the inner peripheral surface ofthe metal casing, there have been proposed monolith-holding elementsprepared by forming an inorganic fiber material into variousconfigurations. The techniques previously proposed for monolith-holdingelements and problems caused thereby are described below.

Japanese Patent Application Laid-open (Kokai) No. 1-240715 (1989)discloses an elastic mat (monolith-holding element) composed of non-shotceramic fiber which is sewed up and compressed in the thicknessdirection. However, such an elastic mat is compressed so as to reduceits thickness only when sewed up at a small sewing pitch. In the casewhere the small sewing pitch is used, there arise problems that theelastic mat is damaged, thereby resulting in lack of elasticity thereof,or that it becomes difficult to conduct delicate processing such asformation of labyrinth structures at opposite joining ends of theelastic mat. Further, there arise problems concerning the fraying,cutting or the like of threads at the sewed end portions of the elasticmat.

British Patent No. 2171180A discloses a mat-like product(monolith-holding element) prepared by vacuum-packing an inorganic fiberin a plastic bag. However, the plastic bag used in such a mat-likeproduct does not have a flexibility. Further, since it is difficult towind the mat-like product around the monolith in a close contact mannerand provide labyrinth structures at opposite joining ends of the plasticbag, there arises a problem concerning sealing properties. Furthermore,there is such a inconvenience that the plastic bag film is likely to bebroken upon transportation or assembling.

U.S. Pat. No. 4,693,338 discloses a monolith-holding element whichcomprises, in combination, a blanket prepared by highly compressing aceramic fiber together with a small amount of a binder, and knittingyarns made of a !4j ceramic fiber. However, such a monolith-holdingelement requires complicated mounting procedures and assembling steps,and therefore, is unsatisfactory.

Japanese Patent Application Laid-open (Kokai) Publication No. 53-2753(1978) discloses a heat-insulating element (monolith-holding element)prepared by compression-molding a fiber-based heat-insulating materialsuch as ceramic fiber, silica fiber, glass fiber or the like togetherwith an organic binder at a compression ratio higher than that in actualuse. However, such a monolith-holding element is deteriorated inelasticity and shows accelerated thermal degradation such as softeningor shrinkage in a high-temperature range, though it exhibits athickness-restoring property in association with thermal decompositionof the organic binder. In addition, since the thickness of themonolith-holding element is thinner than the clearance, the monolith iscaused to move within the casing before the thermal decomposition of theorganic binder, thereby resulting in damage to the monolith.

Japanese Patent Application Laid-open (Kokai) No. 7-77036 (1995)discloses a catalytic converter having a heat-resistant andnon-thermally-expansive ceramic fiber, which comprises a metal casing, aceramic honeycomb catalyst (monolith) accommodated within the metalcasing, and a ceramic fiber mat as a monolith-holding element fitted inan compressed state between an outer surface of the honeycomb catalystand an inner surface of the metal casing. The catalytic converterdisclosed therein includes the ceramic fiber mat having such acompression property which is not largely increased or decreased withina practically used temperature range. However, since no binder iscontained in the afore-mentioned ceramic fiber mat, it is required tohighly compress the ceramic fiber mat upon assembling. This causesdeterioration in working properties and other problems such as breakingof the fiber due to lack of a mechanical strength thereof, scattering ofthe fiber or the like.

Japanese Patent Application Laid-open (Kokai) No. 57-56615 (1982)discloses an apparatus for cleaning exhaust gases, which comprises ametal casing, a ceramic monolith and a sealing element as amonolith-holding element fitted into a clearance between an innercircumferential surface of the metal casing and an outer circumferentialsurface of the monolith. The sealing element disclosed therein iscomposed of a ceramic fiber having a fiber diameter of 6 to 30 μm, andhas a bulk density of 0.1 to 0.35 g/cm³. Further, the sealing element iscompressed such that the ratio between thicknesses before and afterinstallation thereof is within the range of 2.7 to 8.7. However, sincethe sealing element has such a small bulk density and such a largecompression ratio, there arises a problem concerning working propertiesupon assembling.

On the other hand, in the consideration of a heat resistance andcushioning properties of the ceramic fiber, as the monolith-holdingelement there have been proposed sheet elements prepared by blending theceramic fiber with a thermally-expansive material such as vermiculite orthe like for compensating a retaining force thereof.

Japanese Patent Application Laid-open (Kokai) No. 7-127443 (1995)discloses a ceramic honeycomb catalytic converter comprising a metalcasing, a ceramic honeycomb catalyst (monolith), a gripping element(monolith-holding element) composed of a ceramic fiber mat disposed inan compressed state so as to retain the ceramic honeycomb catalyst byits restoring force, and a fixing member for securing the grippingelement at the flowing direction of exhaust gases, the fixing memberbeing possessed in the metal casing. However, the gripping elements madeof a ceramic fiber mat is maintained in a non-compressed condition andtherefore, must be highly compressed upon mounting, whereby there ariseproblems such as the deterioration of working properties and thebreaking of the ceramic fiber itself. Further, there is a tendency thatthe honeycomb catalyst is damaged by a tightening force of the fixingmember which is adapted to prevent a floating movement of the honeycombcatalyst.

In addition, Japanese Patent Publication (Kokoku) No. 58-17335 (1983)discloses a process for producing an integrated catalyst component(catalytic converter) comprising an integrated catalyst (monolith), aceramic fiber molded-element which is separated in a circumferentialdirection of the integrated catalyst and wound around an outercircumferential surface of the integrated catalyst, and a casingsecurely receiving the integrated catalyst therein, which processcomprises the steps of tightening the ceramic fiber molded-elements bymeans of compression rings having a separated structure from its outerperipheral side, covering end portions of the compression rings togetherwith end faces of the ceramic fiber molded-elements by retaining rings,and accommodating the integrated catalyst within the casing. In such acatalytic converter produced according to the afore-mentioned technique,the ceramic fiber molded-elements are prevented from being damaged uponassembling and the end portions thereof is also protected from exhaustgases. However, since the clearance between the integrated catalyst andthe casing, i.e., the clearance between the outer peripheral surface ofthe monolith and the inner peripheral surface of the metal casing isconsiderably fluctuated depending upon the difference in coefficient ofthermal expansion therebetween, it cannot be expected that the ceramicfiber molded-elements exhibits sufficient supporting effects.

Meanwhile, ceramic monoliths composed mainly of cordierite have beenmost widely utilized because they are relatively less expensive andexcellent in thermal dimensional stability. In accordance with thestrengthening of regulations of exhaust gases, efforts have been made toenhance an efficiency of the catalyst by increasing temperaturecharacteristics thereof, and to increase a catalytic surface area anddecrease a heat capacity by reducing a wall thickness of the honeycombcatalyst. However, the reduction in wall thickness of the ceramicmonolith is limited by the production conditions or the mechanicalstrength of the structure. In consequence, it has been recognized thatmetal monoliths are preferable as compared with the ceramic monoliths.

In general, the metal monoliths are composed of aluminum-containingferrite-based stainless steel foils having a thickness of 50 to 100 μm.Since among of the stainless steel foils other than the ferrite-basedfoils, for example austenite-based stainless steel foils show a largethermal expansion or martensite-based stainless steel foils aredifficult to undergo bending, these are not put into practical use.

The ferrite-based stainless steel foils monoliths are prepared bywinding a laminate composed of a flat foil and a corrugated foil into aform of roll and then bonding the opposite ends of the roll together bybrazing to form a cylindrical material. Such metal monoliths have areduced wall thickness which is 1/2 to 1/3 or less of the wall thicknessof the ceramic monoliths. Therefore, nevertheless the metal monolith hasthe same size as that of the ceramic monolith, the metal monolith canshow considerably larger opening rate and surface area, and considerablysmaller heat capacity as compared with those of the ceramic monolith,resulting in achieving a high cleaning efficiency for exhaust gases.Further, the metal monoliths are excellent in dimensional accuracy uponthe production and shows advantages such as a resistance to rupture orfracture. As a result, the metal monoliths are considered to be stablypredominately utilized in future.

In catalytic converters using the afore-mentioned metal monoliths, astructure capable of integrally bonding the monolith to the surroundingmetal casing is adopted in order to ensure bonding of the metal monolithby brazing. However, the afore-mentioned structure using the metalmonolith can show various above-mentioned advantages required for acatalyst carrier as compared with the structure using the ceramicmonoliths. Since a heat generated in the monolith is directlytransferred to the surrounding metal casing, there arises a problem thatthe metal monolith is rapidly cooled during the idling operation.

As techniques concerning catalytic converters using metal monoliths,Japanese Utility Model Application Laid-open (Kokai) No. 1-80620discloses a carrier for a catalyst for cleaning exhaust gases, whichcomprises a sheathed pipe, a metal monolith and a thermally-expansivesheet material interposed between the sheathed pipe and the metalmonolith. However, when the metal monolith is exposed to exhaust gaseshaving a temperature as high as not less than 800° C., the heatresistance thereof is extremely deteriorated. In the case of the metalmonolith disclosed in the above-mentioned Japanese Utility Model KOKAI,there arises such a problem that the honeycomb monolith is readilybuckled by a pressure generated due to the thermal expansion of thethermally-expansive sheet material wound around an outer peripheralsurface of the monolith.

In addition, in order to solve the afore-mentioned problems, JapanesePatent Application Laid-open (Kokai) No. 6-126191 discloses a catalyticconverter using a metal monolith, in which the metal monolith is fixedin the catalytic converter in the same manner as used for fixing theceramic monolith by the monolith-holding element. The catalyticconverter disclosed in the Japanese Patent KOKAI comprises a monolithhaving a honeycomb structure prepared by winding a flat plate and acorrugated plate both made of metal foil into a form of roll, a metalring having a short axial length and bonded to an outer peripheralsurface of the monolith, a casing accommodating the monolith and athermally-expansive sealing element fitted into a clearance between thering and the casing.

In the catalytic converter disclosed in the Japanese Patent KOKAI, sincethe thermal expansion of the monolith becomes larger than that of themetal ring or the like as a monolith-holding element with temperaturerise, a high tightening force is temporarily exerted on the monolith bythe metal ring or the like as a monolith-holding element. Besides, sincethe thermally-expansive sealing element made of a ceramic wool furthercompresses the monolith by means of the metal ring, the monolith islikely to be buckled. As a result, there arises such a problem that themonolith is apt to cause a floating movement within the metal casingduring operation.

Furthermore, WO 94/24425 discloses a mat comprising an integratedcomposite sheet composed of a ceramic fiber and a binder, wherein theceramic fiber contains substantially no shot and has an average fiberlength of about 1 cm to about 10 cm, and the mat has an integralflexible structure and generates a substantially constant pressure overa temperature range of about 20 to about 1,200° C., when the mat isfitted into the clearance. However, such a mat has a problem that itrequires complicated drying processes, and further a ratio of thecompression required upon mounting is considerably large, resulting indeterioration of working properties. In addition, since it is disclosedthat from the consideration of the specific characteristic, i.e., thesubstantially "constant" pressure is generated when mounted into theclearance, it is suggested that the content of the organic binder resinin the mat is small, so that there is a tendency that the fiber in themat is damaged by the frictional contact with the casing upon mounting.Accordingly, it is expected that deterioration in retaining force of themat against the monolith, leakage of exhaust gases from the outerperipheral surface of the monolith are caused.

The present invention has been accomplished in order to solve theafore-mentioned problems. An object of the present invention is toprovide a monolith-holding element and a process for producing themonolith-holding element, in which the monolith-holding element ispreliminarily kept in a compressed state upon assembling of thecatalytic converter, and therefore, has a small thickness so as to bereadily mounted thereto; the monolith-holding element can exhibit athickness-restoring property when heated and a required surface pressurebetween an outer peripheral surface of the monolith and an innerperipheral surface of a metal casing, and does not show anydeterioration in properties after used for a long period of time; andthe monolith-holding element is not corroded even when exposed toexhaust gases having a high-flow rate, and therefore, can maintainsufficient sealing properties.

Another object of the present invention is to provide a catalyticconverter and a process for producing the catalytic converter, in whichthe afore-mentioned monolith-holding element is fitted into a clearancebetween the outer peripheral surface of the monolith and the innerperipheral surface of the metal casing; and the catalytic converter canwithstand the high-temperature of exhaust gases and can stably supportthe monolith even when exposed to severe vibration and impact, so thatthere is no leakage of exhaust gases from the monolith-holding element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a catalytic converteraccording to one embodiment of the present invention;

FIG. 2 is a perspective view showing a manner in which amonolith-holding element is wound around a monolith; and

FIG. 3 is a view schematically showing change in state of an aluminafiber mat up to the production of the monolith-holding element.

DISCLOSURE OF THE INVENTION

As a result of intense studies by the present inventors to accomplishthe afore-mentioned objects, it has been found that by preparing themonolith-holding element from an alumina fiber mat which produces apredetermined restoring force when compressed so as to reduce itsthickness to that corresponding to a clearance between an outerperipheral surface of the monolith and an inner peripheral surface ofthe metal casing, the monolith is prevented from being buckled and themonolith-holding element can show sufficient supporting effects. Thepresent invention has been attained on the basis of the finding.

That is, the present invention relates to a monolith-holding element, aprocess for producing the said monolith-holding element, a catalyticconverter using a monolith and a process for producing the saidcatalytic converter. Various aspects of the present invention aredescribed below.

In a first aspect of the present invention, there is provided amonolith-holding element for use in a catalytic converter comprising acylindrical monolith supporting a catalyst for cleaning exhaust gasesthereon, a metal casing accommodating the monolith and connected toexhaust pipes, and the monolith-holding element fitted into a clearancebetween an outer surface of the monolith and an inner surface of themetal casing, which monolith-holding element comprises an alumina fibermat compressed in the thickness direction thereof and an organic binderuniformly dispersed in the alumina fiber mat and capable of beingdissipated by thermal decomposition thereof, the monolith-holdingelement being produced by a process comprising (I) the first step ofimpregnating the alumina fiber mat with a solution containing theorganic binder, (II) the second step of compressing the alumina fibermat impregnated with the organic binder-containing solution in thethickness direction, and (III) the third step of removing a solvent ofthe organic binder-containing solution while maintaining the thicknessof the compressed alumina fiber mat, wherein the thickness of themonolith-holding element produced in the third step is 1 to 1.5 timesthe thickness of the alumina fiber mat compressed in the second step,and when the organic binder is thermally decomposed, themonolith-holding element exhibits a thickness-restoring property withopposite surfaces thereof kept in an open state, and a restorationsurface pressure of the monolith-holding element being kept under such acompressed condition that the thickness thereof is reduced to thatcorresponding to the clearance, is in the range of 0.5 to 30 kg/cm².

In a second aspect of the present invention, there is provided amonolith-holding element for use in a catalytic converter comprising acylindrical monolith supporting a catalyst for cleaning exhaust gasesthereon, a metal casing accommodating the monolith and connected toexhaust pipes and the monolith-holding element fitted into a clearancebetween an outer peripheral surface of the monolith and an innerperipheral surface of the metal casing, which monolith-holding elementcomprises an alumina fiber mat compressed in the thickness directionthereof and an organic binder uniformly dispersed in the alumina fibermat and capable of being dissipated by thermal decomposition thereof,wherein the monolith-holding element maintained in a compressedcondition by the organic binder has a bulk density of 0.1 to 0.5 g/cm³,when the organic binder is thermally decomposed, the monolith-holdingelement exhibits a thickness-restoring ratio of 2 to 10 times withopposite surfaces thereof kept in an open state, and a restorationsurface pressure of the monolith-holding element kept under such acompressed condition that the thickness thereof is reduced to thatcorresponding to the clearance, is in the range of 0.5 to 30 kg/cm².

In a third aspect of the present invention, there is provided a processfor producing a monolith-holding element for use in a catalyticconverter comprising a cylindrical monolith supporting a catalyst forcleaning exhaust gases thereon, a metal casing accommodating themonolith and connected to exhaust pipes and the monolith-holding elementfitted into a clearance between an outer peripheral surface of themonolith and an inner peripheral surface of the metal casing, whichprocess comprises (I) the first step of impregnating an alumina fibermat having a bulk density of 0.05 to 0.20 g/cm³ with a solutioncontaining an organic binder capable of being dissipated by thermaldecomposition thereof, (II) the second step of compressing the aluminafiber mat impregnated with the organic binder-containing solution in thethickness direction so as to reduce a thickness thereof by 1/2 to 1/15times, and (III) the third step of removing a solvent of the organicbinder-containing solution while maintaining the thickness of thecompressed alumina fiber mat, wherein when the organic binder isthermally decomposed, the monolith-holding element exhibits athickness-restoring property with opposite surfaces thereof kept in anopen state, and a restoration surface pressure of the monolith-holdingelement kept under such a compressed condition that the thicknessthereof is reduced to that corresponding to the clearance, is in therange of 0.5 to 30 kg/cm².

In a fourth aspect of the present invention, there is provided acatalytic converter comprising a cylindrical monolith supporting acatalyst for cleaning exhaust gases thereon, a metal casingaccommodating the monolith and connected to exhaust pipes and amonolith-holding element fitted into a clearance between an outersurface of the monolith and an inner surface of the metal casing, themonolith-holding element comprising a alumina fiber mat compressed inthe thickness direction thereof and an organic binder uniformlydispersed in the alumina fiber mat and capable of being dissipated bythermal decomposition, the monolith-holding element being produced by aprocess comprising (I) the first step of impregnating the alumina fibermat with a solution containing the organic binder, (II) the second stepof compressing the alumina fiber mat impregnated with the organicbinder-containing solution in the thickness direction, and (III) thethird step of removing a solvent of the organic binder-containingsolution while maintaining the thickness of the compressed alumina fibermat, wherein the thickness of the monolith-holding element produced inthe third step is 1 to 1.5 times the thickness of the alumina fiber matcompressed in the second step, and when the organic binder is thermallydecomposed, the monolith-holding element exhibits a thickness-restoringproperty with opposite surfaces thereof kept in an open state, and arestoration surface pressure of the monolith-holding element whoseopposite surfaces are kept in a fixed condition is in the range of 0.5to 30 kg/cm².

In a fifth aspect of the present invention, there is provided acatalytic converter comprising a cylindrical monolith supporting acatalyst for cleaning exhaust gases thereon, a metal casingaccommodating the monolith and connected to exhaust pipes and themonolith-holding element fitted into a clearance between an outersurface of the monolith and an inner surface of the metal casing, themonolith-holding element being prepared by compressing a alumina fibermat having a bulk density of 0.05 to 0.20 g/cm³ in the thicknessdirection thereof and containing an organic binder uniformly dispersedin the alumina fiber mat and capable of being dissipated by thermaldecomposition, wherein the monolith-holding element is maintained in acompressed condition by the organic binder and has a bulk density of 0.1to 0.5 g/cm³, and when the organic binder is thermally decomposed, themonolith-holding element exhibits a thickness-restoring ratio of 2 to 10times with opposite surfaces thereof kept in an open state, and arestoration surface pressure of the monolith-holding element whoseopposite surfaces are kept in a fixed condition is in the range of 0.5to 30 kg/cm².

In a sixth aspect of the present invention, there is provided acatalytic converter comprising a cylindrical monolith supporting acatalyst for cleaning exhaust gases thereon, a metal casingaccommodating the monolith and connected to exhaust pipes and themonolith-holding element fitted into a clearance between an outersurface of the monolith and an inner surface of the metal casing,wherein the monolith comprises a ferrite-based stainless steel foil andprovided with a honeycomb structure, the monolith is supported directlyby the monolith-holding element comprising an alumina fiber matcompressed in the thickness direction and an organic binder uniformlydispersed in the alumina fiber mat and capable of being dissipated bythermal decomposition, and when the organic binder is thermallydecomposed, a restoration surface pressure of the monolith-holdingelement being kept under such a compressed condition that the thicknessthereof is reduced to that corresponding to the clearance, is in therange of 0.1 to 4.0 kg/cm².

In a seventh aspect of the present invention, there is provided aprocess for producing a catalytic converter comprising a cylindricalmonolith supporting a catalyst for cleaning exhaust gases thereon, acylindrical metal casing connected to exhaust pipes and amonolith-holding element fitted into a clearance between an outersurface of the monolith and an inner surface of the cylindrical metalcasing, the monolith being composed of a ferrite-based stainless steelfoils and having a honeycomb structure, which process comprises (I) thefirst step of winding the monolith-holding element around the outerperipheral surface of the cylindrical monolith, (II) the second step of,after accommodating the cylindrical monolith around which themonolith-holding element is wound, in the cylindrical metal casinghaving a two-piece structure composed of an upper shell member and alower shell member, bringing the upper and lower shell members intomating contact with each other and connecting the upper and lower shellmembers together by welding mating peripheral portions thereof, whereinthe said monolith-holding element comprises an alumina fiber matcompressed in the thickness direction and an organic binder uniformlydispersed in the alumina fiber mat, and when the organic binder isthermally decomposed, the restoration surface pressure of themonolith-holding element kept under such a compressed condition that thethickness thereof is reduced to that corresponding to the clearance, isin the range of 0.1 to 4.0 kg/cm².

In an eighth aspect of the present invention, there is provided aprocess for producing a catalytic converter, which comprisesaccommodating a cylindrical monolith supporting a catalyst for cleaningexhaust gases thereon, into a metal casing connected to exhaust pipes,and fitting a monolith-holding element into a clearance between an outersurface of the monolith and an inner surface of the metal casing, themonolith being composed of a ferrite-based stainless steel foil andhaving a honeycomb structure and being supported directly themonolith-holding element, the monolith-holding element being produced bya process comprising (I) the first step of impregnating the aluminafiber mat with a solution containing the organic binder, (II) the secondstep of compressing the alumina fiber mat impregnated with the organicbinder-containing solution in the thickness direction, and (III) thethird step of removing a solvent of the organic binder-containingsolution while maintaining the thickness of the compressed alumina fibermat, wherein when the organic binder is thermally decomposed, arestoration surface pressure of the monolith-holding element kept undersuch a compressed condition that the thickness thereof is reduced tothat corresponding to the clearance, is in the range of 0.1 to 4.0kg/cm².

The monolith-holding element, the process for the production of themonolith-holding element, the catalytic converter using the monolith andthe process for the production of the catalytic converter in accordancewith the present invention are described in detail below by referring tothe accompanying drawings.

The monolith-holding element according to the present invention isapplicable to the catalytic converter which comprises a monolith havinga cylindrical shape and supporting a catalyst for cleaning exhaust gasesthereon, a metal casing accommodating the monolith and connected toexhaust pipes, and the monolith-holding element fitted into a clearanceformed between an outer peripheral surface of the monolith and an innerperipheral surface of the metal casing. The monolith-holding element canexhibit a thickness-restoring property due to the thermal decompositionof an organic binder contained therein when it is exposed tohigh-temperature exhaust gases from an internal combustion engine, uponwhich the monolith-holding element can exert a surface pressure onto theouter peripheral surface of the monolith and the inner peripheralsurface of the metal casing, whereby the monolith is held in placewithin the metal casing.

The terminology used herein to explain the present invention are definedas follows.

The base mat means a mat composed of an alumina fiber, and thecompressed mat means the base mat which is compressed in the thicknessdirection thereof. The mat used in the present invention is a non-wovenfiber-like accumulated material composed of alumina fibers havingapproximately uniform thickness, and involves those called a blanket ora block.

The ordinary-state thickness represents a thickness of the base matwhich is free from a compression force in its thickness direction(thickness (A) shown in FIG. 3(a)).

The compressed thickness represents a thickness of the compressed mat(thickness (B) shown in FIG. 3(b)).

The ordinary-state thickness of the monolith-holding element representsthe thickness of the monolith-holding element (molded product) beforemounted to the catalytic converter (thickness (C) shown in FIG. 3(c)).

The clearance represents a space defined between the outer peripheralsurface of the monolith and the inner peripheral surface of the metalcasing (reference numeral (D) in FIG. 3(d)).

Incidentally, the monolith-holding element is hereinafter referred tomerely as a "holder".

First, the holder according to the first aspect of the present inventionis explained by referring to FIG. 3.

In FIG. 3, the afore-mentioned holder represented by reference numeral 3comprises the compressed mat and the organic binder uniformly dispersedin the compressed mat and capable of being dissipated from the mat bythe thermal decomposition thereof. The process for the production of theholder comprises (I) the first step of impregnating the base mat with asolution containing the organic binder, (II) the second step ofcompressing the base mat impregnated with the organic binder-containingsolution in its thickness direction, and (III) the third step ofremoving a solvent of the organic binder-containing solution whilemaintaining the thickness (B) of the compressed base mat, in which thethickness (C) of the holder 3 obtained in the third step is 1 to 1.5times the compressed thickness (B) of the compressed mat obtained in thesecond step, the mat can exhibit a thickness-restoring property with theopposite surfaces thereof being kept in an open state when the organicbinder contained in the mat is thermally decomposed, and the surfacepressure of the holder under such a compressed condition that the holderhas a thickness corresponding to the clearance, is in the range of 0.5to 30 kg/cm². The compressed condition in which the holder has athickness corresponding to the clearance, is hereinafter referred tomerely as "fixed condition of opposite surfaces."

The alumina fiber constituting the base mat is selected from fiberscapable of exhibiting the thickness-restoring property when the mat isin the form of the compressed mat, that is, those capable of exhibitinga resiliency against the compression force in the thickness direction ofthe mat. The alumina fiber has generally a fiber diameter of 1 to 50 μmand a fiber length of 0.5 to 500 mm, preferably a fiber diameter of 1 to10 μm and a fiber length of 0.5 to 300 mm.

Typical examples of the alumina fiber are an alumina/silica-basedcrystalline short fiber having a silica content of not more than 5% byweight, namely an alumina content of not less than 95% by weight, andother ordinarily used alumina fibers containing 70 to 95% by weight ofalumina and the remainder consisting of silica. Especially, mullitefibers containing 72% by weight of alumina is preferred, because it isexcellent in a high-temperature stability and an elasticity.

The afore-mentioned alumina/silica-based polycrystalline fiber generallycalled as an alumina fiber, is superior in heat resistance to anamorphous ceramic fiber which has been used in the condition where amaximum temperature is approximately 1,200° C., and therefore, canwithstand a temperature of 1,500 to 1,600° C. which is high enough ascompared with that of exhaust gases from the internal combustion engine.Besides, the compressed mat composed of the alumina fiber has asufficient elasticity and is considerably reduced in heat deteriorationsuch as softening and shrinking observed in the case of the ceramicfiber, thereby providing a holder 3 having a low deterioration with thepassage of time.

The base mat may be made of a single kind of the afore-mentioned aluminafibers or a mixture of any two or more kinds of the alumina fibers.Further, the base mat may be of a laminate comprising a plurality ofidentical or different mats each made of a single kind of aluminafibers. The bulk density of the base mat is generally in the range of0.05 to 0.2 g/cm³, preferably 0.05 to 0.15 g/cm³. When the bulk densityof the base mat is less than 0.05 g/cm³, the compression ratio becomeslarge. On the other hand, when the bulk density of the base mat is morethan 0.2 g/cm³, the holder often exhibits an insufficientthickness-restoring force.

The holder 3 according to the present invention contains an organicbinder uniformly dispersed in the mat compressed in its thicknessdirection. The bonding force of the organic binder suppresses therestoration to its original thickness.

As the organic binder, any binders composed of an organic compound canbe usable in the present invention without particular limitations, asfar as the binders can maintain the compressed thickness (B) of thecompressed mat at an ordinary temperature, and the thermal decompositionthereof permits restoration of the original thickness of the mat. It ispreferred that the organic binder be readily thermally decomposed anddissipated (destroyed) from the mat at a temperature at which the holder3 is intended to be used. However, if the organic binder no longer has afunction as a binder at the intended temperature to cause the holder torestore its original thickness, it is not necessary that the organicbinder is dissipated from the mat by the thermal decomposition. Further,since the holder 3 is exposed generally to a temperature of not lessthan 600° C. or to a temperature of 900 to 1,000° C. for ahigh-temperature use, it is preferred that the organic binder bethermally decomposed for a short period of time so as to lose itsfunction as a binder at a temperature of about 600° C. or lower. Morepreferably, the organic binder is dissipated at the temperature rangefrom the mat upon the thermal decomposition.

As the organic binders, various rubbers, water-soluble polymercompounds, thermoplastic resins, thermosetting resins or the like areexemplified. Examples of the rubbers include natural rubbers; acrylicrubbers such as copolymers of ethyl acrylate and chloroethyl-vinylether, copolymers of n-butyl acrylate and acrylonitrile or the like;nitrile rubbers such as copolymers of butadiene and acrylonitrile or thelike; butadiene rubbers or the like. Examples of the water-solublepolymer compounds include carboxymethyl cellulose, polyvinyl alcohol orthe like. Examples of the thermoplastic resins include acrylic resins inthe form of homopolymers or copolymers of acrylic acid, acrylic acidesters, acrylamide, acrylonitrile, methacrylic acid, methacrylic acidesters or the like; an acrylonitrile-styrene copolymer; anacrylonitrile-butadiene-styrene copolymer or the like. Examples of thethermosetting resins include bisphenol-type epoxy resins, novolac-typeepoxy resins or the like.

The afore-mentioned organic binders may be used in the form of anaqueous solution, a water-dispersed emulsion, a latex or a solutionusing an organic solvent. These organic binders are hereinafter referredto generally as a "binder liquid".

As the afore-mentioned binder liquid, commercially available binders canbe used as such or in the form of a solution diluted with water or thelike. Incidentally, the organic binders are not necessarily used singly,but the mixture of any two or more thereof can be also used.

Among the afore-mentioned organic binders, the compounds selected fromthe group consisting of synthetic rubbers such as acrylic rubbers,nitrile rubbers or the like; water-soluble polymer compounds such ascarboxymethyl cellulose, polyvinyl alcohol or the like; and acrylicresins are preferable. Among them, acrylic rubbers, nitrile rubbers,carboxymethyl cellulose, polyvinyl alcohol, acrylic resins other thanthe acrylic rubbers are particularly preferred. These organic bindersare readily available and can be readily treated or handled during thepreparation of the binder liquid and the impregnation of the binderliquid into the compressed mat. Further, these organic binders not onlycan exhibit a sufficient thickness-retaining force even at a relativelylow content thereof, but also can be easily thermally decomposed underthe intended temperature condition, thereby providing the holder 3having an excellent flexibility and a high mechanical strength.

The content of the organic binder in the compressed mat is notparticularly restricted but may be appropriately determined dependingupon a kind of the fiber used in the compressed mat, the compressedthickness (B) of the compressed mat, a resiliency of the compressed matagainst the compression force applied thereto or the like. The contentis generally in the range of 10 to 30 parts by weight (on the solidbasis) based on 100 parts by weight of the alumina fiber. When thecontent of the organic binder in the compressed mat is less than 10parts by weight, it is difficult to maintain the compressed thickness(B) of the compressed mat. However, in order to maintain the compressedthickness (B) of the compressed mat, it is not necessary that thecontent of the organic binder contained in the mat exceeds 30 parts byweight. When the content of the organic binder is too high, not only theproduction cost of the holder is increased, but also it becomesdifficult to thermally decompose an entire amount of the organic binder.From this viewpoint, the content of the organic binder in the compressedmat is preferably in the range of 15 to 25 parts by weight.

The holder 3 according to the present invention may be produced by theprocess comprising (I) the first step of impregnating the base mat withthe organic binder, (II) the second step of compressing the base matimpregnated with a solution containing the organic binder in itsthickness direction, and (III) the third step of removing a solvent ofthe organic binder-containing solution while maintaining the compressedthickness (B) of the compressed base mat. It is preferred that theordinary-state thickness (A) of the base mat used in the first step is 2to 10 times the thickness (C) of the holder 3.

The compressed thickness (B) of the compressed mat obtained in thesecond step may be determined depending upon the thickness-restoringproperty, the resiliency against the compression force applied, thesurface pressure for restoration to the aimed holder 3, the gas sealingproperties or the like. In order to accomplish the afore-mentionedrestoration ratio, the ratio of the compressed thickness (B) of thecompressed mat to the ordinary-state thickness (A) of the base mat isgenerally about 1/1.25 or less, preferably 1/2 to 1/15. Further, thethickness (C) of the holder 3 obtained in the third step is 1 to 1.5times the compressed thickness (B) of the compressed mat obtained in thesecond step. Incidentally, the details of the respective steps aredescribed below in explaining the process of the production of theholder 3 according to the third aspect of the present invention.

As will be understood from the afore-mentioned description, in theholder 3 according to the present invention, the specific material, thespecific ordinary-state thickness (A) or the like of the base mat may bedetermined according to the elasticity in the thickness direction, theclearance (D) of the catalytic converter, the surface pressure forrestoration to the aimed holder 3, the gas sealing properties or thelike.

The thus-constructed holder 3 according to the present invention canexhibit a thickness-restoring property with the opposite surfaces keptin an open state, when the organic binder contained therein is thermallydecomposed. In the case where the opposite surfaces of the holder ismaintained in the fixed condition, the restoration surface pressure atthe opposite surfaces is adjusted to 0.5 to 30 kg/cm², preferably 0.5 to8 kg/cm².

The restoration surface pressure of the holder 3 cannot be directlymeasured after the holder is mounted to the catalytic converter, andtherefore, may be determined from a value for the molded productmeasured before mounting to the catalytic converter. Such a restorationsurface pressure can be measured by one of the methods set forth below.

(1) Direct method in which one surface of the molded product bearsagainst a stationary face plate and the other surface thereof is broughtinto contact with a measuring face plate, and thereafter, the organicbinder contained in the molded product is thermally decomposed whilemaintaining the contact of both the surfaces with the face plates, uponwhich a surface pressure on the measuring face plate is directlymeasured.

(2) Indirect method in which after the organic binder contained in themolded product is thermally decomposed with the opposite surfaces keptin an open state to permit the molded product to be restored, the moldedproduct is compressed by means of a face plate until reaching thethickness (C) before the thermal decomposition, upon which a pressureapplied onto the face plate to conduct the compression is measured as arestoration surface pressure of the holder.

Among the afore-mentioned two methods, the latter indirect method ispreferred because of its simplicity. In Examples described later of thepresent invention, a compression force required for the production ofthe compressed mat is used as an index for representing the restorationsurface pressure.

The reason why the restoration surface pressure of the holder isadjusted to the afore-mentioned range is as follows. That is, when therestoration surface pressure is less than 0.5 kg/cm², there is alikelihood that the monolith 1 is not surely secured to the metalcasing. On the other hand, the restoration surface pressure more than 30kg/cm² is not necessary for the fixing of the monolith 1. Therestoration surface pressure within the afore-mentioned range does notpose any problem to achieve a good gas sealing properties around anouter periphery of the monolith 1.

The restoring property of the holder can be also determined by the ratioof the thickness of the holder 3, which is obtained after the thermaldecomposition of the organic binder when the opposite surfaces thereofis kept in an open state, to the thickness (C) thereof, i.e., athickness-restoring ratio with the opposite surfaces kept in an openstate. Such a thickness-restoring ratio is preferably in the range of 2to 10 times.

When the thicknesses of the holder before and after the thermaldecomposition of the organic binder are represented by d₀ mm and d mmrespectively, the thickness-restoring ratio of the holder 3 is definedby the following formula (I):

    Thickness-restoring ratio=d/d.sub.0                        (I)

Further, the holder 3 of the present invention which is constructed ofthe afore-mentioned material, has a bulk density of 0.1 to 0.5 g/cm³,preferably 0.2 to 0.4 g/cm³. Furthermore, the holder preferably has aunit tensile strength of 10 to 40 kg/cm² and a tensile modulus of 200 to700 kg/cm².

The holder 3 having the afore-mentioned properties can securely hold themonolith without buckling when mounted to the catalytic converter,because of its restoration surface pressure exhibited after the thermaldecomposition.

The thickness (C) of the holder 3 according to the present invention maybe determined depending upon the clearance (D) of the catalyticconverter. In general, in the case where the clearance (D) is from 2 to8 mm, preferably from 3 to 6 mm, it is suitable that the thickness (C)of the corresponding holder 3 is in the range of 3 to 10 mm. Thethickness of the holder 3 is 1.0 to 2.0 times, preferably 1.0 to 1.6times the clearance (D).

Next, the holder according to the second aspect of the present inventionis described by referring to FIG. 3.

In FIG. 3, the holder represented by reference numeral 3 is composed ofthe base mat compressed in the thickness direction and the organicbinder uniformly dispersed in the base mat, and capable of dissipatingupon the thermal decomposition thereof. The holder is maintained in acompressed state by the organic binder, and has a bulk density of 0.1 to0.5 g/cm³, a thickness-restoring ratio of 2 to 10 times when the organicbinder is thermally decomposed while keeping the opposite surfaces ofthe holder in an open state, and a restoration surface pressure of 0.5to 30 kg/cm² when the opposite surfaces are kept in the fixed condition.

The afore-mentioned holder 3 may be made of the same base material asthat used for the holder 3 according to the first aspect of the presentinvention. In addition, the holder can be also produced, for example, bythe same process as that used for the holder according to the firstaspect of the present invention. In this case, it is important that theholder 3 is maintained in a compressed state by the organic binder, andhas a bulk density of 0.1 to 0.5 g/cm³, a thickness-restoring ratio of1.25 to 10 times when the organic binder is thermally decomposed whilekeeping the opposite surfaces in an open state, and a restorationsurface pressure of 0.5 to 30 kg/cm² when the opposite surfaces are keptin the fixed condition.

It is preferred that other properties of the holder 3 is the same asthose of the holder 3 according to the first aspect of the presentinvention. Specifically, the ordinary-state thickness (A) of the basemat is preferably 2 to 10 times the thickness (C) of the holder 3. Inaddition, the organic binder is preferably made of at least one materialselected from the group consisting of acrylic rubbers, nitrile rubbers,carboxymethyl cellulose, polyvinyl alcohol and acrylic resins other thanthe acrylic rubbers. The content of the organic binder is preferably 10to 30 parts by weight based on 100 parts by weight of the alumina fibermat. Further, similarly to the holder according to the first aspect ofthe present invention, the holder preferably has a restoring surfacepressure of 0.5 to 8 kg/cm² when the opposite surfaces of the holder iskept in the fixed condition, a unit tensile strength of 10 to 40 kg/cm²and a tensile modulus of 200 to 700 kg/cm².

Next, the process for producing the holder according to the third aspectof the present invention, is described by referring to FIG. 3.

The afore-mentioned process comprises (I) the first step of impregnatinga base mat having a bulk density of 0.05 to 0.2 g/cm³, preferably 0.05to 0.15 g/cm³, with a solution containing an organic binder capable ofdissipating by the thermal decomposition thereof, (II) the second stepof compressing the base mat impregnated with the organicbinder-containing solution in its thickness direction to form acompressed base mat having a thickness 1/2 to 1/15 time that of the basemat, and (III) the third step of removing a solvent of the organicbinder-containing solution while maintaining the thickness (B) of thecompressed base mat, wherein the holder 3 has a thickness-restoringproperty when the organic binder is thermally decomposed while keepingthe opposite surfaces of the mat in an open state, and a restorationsurface pressure of 0.5 to 30 kg/cm² when the opposite surfaces are keptin the fixed condition.

In the afore-mentioned process, the same base mat and organic bindersolution as those described above can be used.

In the afore-mentioned first step, the base mat is impregnated with theorganic binder solution.

The base mat has a bulk density of 0.05 to 0.2 g/cm³, preferably 0.05 to0.15 g/cm³, and the same ordinary-state thickness as that describedabove. Also, the organic binder solution is suitably used in the form ofan aqueous solution, an aqueous dispersion-type emulsion, a latex or asolution using an organic solvent, which contain as effectivecomponents, acrylic rubbers, nitrile rubbers, carboxymethyl cellulose,polyvinyl alcohol or acrylic resins. The concentration of the organicbinder in the solution may be adequately determined depending upon themethod of impregnating the organic binder solution into the base mat,the amount of the organic binder to be retained in the mat or the like,but is generally in the range of 2 to 50% by weight.

As the method for impregnating the organic binder solution into the basemat, there can be used a method of immersing the base mat in the organicsolution or a method of spraying the organic binder solution onto thebase mat. In the case where the base mat to be treated has a largeordinary-state thickness (A), it is preferable to use the immersingmethod. On the other hand, in the case where the base mat to be treatedhas a small ordinary-state thickness (A), it is preferred to use thespraying method. In the case the impregnation is conducted by thespraying method, the organic binder solution of an emulsion-typeexhibiting little stringing phenomenon is suitably used.

Alternatively, the impregnation of the base mat can be carried out bydispersing and suspending bulky alumina fiber used as a startingmaterial in an organic binder solution and then collecting the suspendedalumina fiber on a screen plate or a porous plate by filtering orpaper-making techniques to form a flat accumulated material of thealumina fiber thereon. In this method, the alumina fiber is apt to becut during the dispersion and suspension, so that the care must be takenin the dispersing and suspending operations. Therefore, it is preferredthat the organic binder solution used has a low viscosity.

In the afore-mentioned second step, the base mat impregnated with theorganic binder solution is compressed in the thickness directionthereof, thereby determining the thickness (C) of the aimed holder 3 andcontrolling the content of the organic binder in the holder.

As the compressing means, press plates or press rollers can be used topress the base mat. The press plate may comprise two liquid-permeableplate members. Typical examples of the press plates include punchingmetals, resin nets, metal nets (meshes), porous plates, air-permeableplates or the like. These compressing means are preferably used incombination with a means for absorbing the binder solution. Further, thecompressing method is preferably carried out in combination with acentrifugal liquid-removing method. Specifically, the removal of liquidfrom a surface of the compressed mat is promoted to prevent the organicbinder from being heterogeneously distributed thereon and concentratedlylocated on the surface thereof. This results in effectively preventingvarious inconveniences due to the use of the organic binder, forexample, adherence of the binder to an drying apparatus used for removal(drying) of the solvent in the subsequent third step, or the like.

In the afore-mentioned third step, the solvent contained in the organicbinder solution is removed while maintaining the compressed thickness(B) of the compressed mat obtained in the second step, to prevent thecompressed mat from temporarily restoring its thickness in undried statethereof and avoid the organic binder from being heterogeneouslydistributed therein and concentratedly located on the surface thereof.

It is preferred that the third step is rapidly carried out by supplyinga hot air under such a temperature condition that the organic binder isnot caused to be degraded and decomposed. This is because the organicbinder contained in the organic binder solution is prevented from beingprecipitated, thereby further avoiding the biased distribution of theorganic binder in the mat. As a result, the holder 3 having a largerestoration surface pressure can be obtained.

Similarly to the holder according to the second aspect of the presentinvention, it is required that the holder 3 according to the thirdaspect of the present invention exhibits a thickness-restoring propertyin an open state of the opposite surfaces thereof when the organicbinder is subjected to thermal decomposition, and has a restorationsurface pressure of 0.5 to 30 kg/cm², preferably 0.5 to 8 kg/cm² whenthe opposite surfaces thereof are kept in the fixed condition. Further,the other properties of the holder 3 according to the third aspect ofthe present invention preferably are the same as those of the holders 3according to the first and second aspects of the present invention.

Next, the catalytic converter according to the fourth aspect of thepresent invention is described by referring to FIGS. 1 to 3.

The catalytic converter shown in FIG. 1 comprises a monolith 1 of acylindrical shape for supporting a catalyst for cleaning exhaust gasesthereon, a metal casing 2 accommodating the monolith and connected toexhaust pipes and a holder 3 inserted into a clearance (D) between anouter surface of the monolith 1 and an inner surface of the metal casing2. The holder 3 is composed of a base mat compressed in the thicknessdirection thereof and an organic binder uniformly dispersed in the basemat and capable of being dissipated when subjected to thermaldecomposition. The process for the production of the holder 3 comprises(I) the first step of impregnating the base mat with the organic bindersolution, (II) the second step of compressing the base mat impregnatedwith the organic binder solution in its thickness direction to athickness (B), and (III) the third step of removing a solvent of theorganic binder solution while maintaining the ordinary-state thickness(B) of the compressed base mat, wherein the thickness (C) of the holder3 obtained in the third step is 1 to 1.5 times the compressed thickness(B) of the compressed base mat obtained in the second step, and when theorganic binder is subjected to thermal decomposition, the holder 3exhibits a thickness-restoring property in an open state of the oppositesurfaces thereof and has a restoration surface pressure of 0.5 to 30kg/cm² in a fixed condition of the opposite surfaces.

Incidentally, in the illustrated catalytic converter, the monolith 1having a honeycomb structure is used as a preferred embodiment thereof.In the figures, reference numeral 2a denotes an upper shell member ofthe metal casing; reference numeral 21a denotes a flange portion of theupper shell member 2a; reference numeral 2b denotes a lower shell memberof the metal casing; reference numeral 21b denotes a flange portion ofthe lower shell member 2b; reference numerals 22a and 22b denote boltholes for fixing the catalytic converter onto a vehicle body such aschassis; and reference numerals 4 and 5 denote openings at which thecatalytic converter is coupled with exhaust pipes.

The holder 3 used in the catalytic converter according to the fourthaspect of the present invention, may be the same as the holders 3according to the first and second aspects, which can be obtainedaccording to the third aspect of the present invention. Especially, inthe afore-mentioned catalytic converter, the ordinary-state thickness(A) of the base mat is preferably 2 to 10 times the thickness (C) of theholder 3.

The catalytic converter according to the fourth aspect of the presentinvention comprises a monolith 1 of a cylindrical shape for supporting acatalyst for cleaning exhaust gases thereon, a metal casing 2accommodating the monolith and connected to exhaust pipes and a holder 3inserted into a clearance (D) between an outer surface of the monolith 1and an inner surface of the metal casing 2.

In the afore-mentioned catalytic converter, it is not required that theholder 3 has the same thickness as the clearance (D), but the holderhaving a slightly larger thickness than the clearance (D) can be alsomounted thereinto. However, when the thickness of the holder is toolarge or when the holder is less slidable on the metal casing, therearises such an inconvenience that the holder projects outwardly and isinterposed between mating surfaces of the flange portions 21a and 21b,so that the mating surfaces cannot be welded to each other. Inconsequence, the thickness (C) of the holder 3 is preferably 1.0 to 2.0times the clearance (D) of the catalytic converter. In addition, sincethere is a likelihood that the compressed mat obtained in the secondstep cannot exhibit a sufficient restoring force due to compressionhysteresis, it is preferred that the compressed thickness (B) of thecompressed mat is larger than the clearance (D).

Further, it is preferred that when mounted to the catalytic converter,the holder 3 has an initial bulk density of 0.18 to 0.8 g/cm³,preferably 0.2 to 0.6 g/cm³, and an initial surface pressure of 0 to 8kg/cm². In addition, the restoration surface pressure of the holder 3 inthe clearance (D) is preferably in the range of 0.5 to 8 kg/cm² upon thethermal decomposition of the organic binder, which amounts to 50 to 90%or 110 to 150% of the initial surface pressure.

In the catalytic converter illustrated in the figures, the metal casing2 has a two-piece clamshell structure comprising the upper half shellmember 2a and the lower half shell member 2b which are integrallyconnected with each other. The upper and lower shell members 2a and 2bare provided with the flange portions 21a and 21b, respectively, whichfunction as mating surfaces when the upper and lower shell members areconnected to each other by welding. Further, the lower shell member 2bof the metal casing is provided with openings 4 and 5 for connecting thecatalytic converter with exhaust pipes. Alternatively, a casing of astuffing box structure, which is preliminarily formed into a cylinder soas to accommodate the monolith 1 therein, can be also used as the metalcasing 2.

The metal casing 2 has an inner diameter larger than an outer diameterof the monolith 1 such that the clearance (D) of about 3 to about 6 mm,usually about 4 mm, is formed therebetween to ensure the insertion ofthe holder 3. The holder 3 has the thickness (C) which is 1.0 to 2.0times, preferably 1.0 to 1.6 times the clearance (D), and is mountedover an entire outer peripheral surface of the monolith 1. However, itis not necessarily required that the holder 3 extends over the entireouter peripheral surface. Instead, a strip-shaped holder can be mountedonly around longitudinally-center portion of the outer peripheralsurface of the monolith or two strip-shaped holders can be mountedaround opposite ends of the outer peripheral surface of the monolith.Further, a ceramic fiber mat made of alumina/silica-based fiber having alower heat resistance than that of the alumina fiber can be usedtogether with the holder according to the present invention, such thatthe ceramic fiber mat placed on the side of the low-temperature metalcasing is laminated over the holder placed on the side of the honeycombmonolith.

The afore-mentioned catalytic converter can be basically producedaccording to the below-mentioned process for the production of acatalytic converter according to the seventh aspect of the presentinvention.

Next, the catalytic converter according to the fifth aspect of thepresent invention is described by referring to FIGS. 1 to 3.

The catalytic converter according to the fifth aspect of the presentinvention is substantially the same as that according to the fourthaspect of the present invention except that the production method forthe holder used therein is not limited to particular ones. In theafore-mentioned catalytic converter, the holder 3 may be produced bycompressing a base mat having a bulk density of 0.05 to 0.2 g/cm³,preferably 0.05 to 0.15 g/cm³, in its thickness direction. The organicbinder capable of being dissipated by thermal decomposition is uniformlydispersed in the compressed mat, whereby the compressed mat ismaintained in a compressed state and has a bulk density of 0.1 to 0.5g/cm³, preferably 0.2 to 0.4 g/cm³. When the organic binder is thermallydecomposed, the holder exhibits a thickness-restoring ratio of 2 to 10times in an open state of the opposite surface thereof and a restorationsurface pressure of 0.5 to 30 kg/cm² in a fixed condition of theopposite surfaces thereof.

The afore-mentioned catalytic converter comprises a monolith 1 of acylindrical shape for supporting a catalyst for cleaning exhaust gasesthereon, a metal casing 2 accommodating the monolith and connected toexhaust pipes and the holder 3 inserted into a clearance (D) between anouter surface of the monolith 1 and an inner surface of the metal casing2. In this case, the holder 3 has the thickness (C) which is 1.0 to 2.0times the clearance (D) formed between the outer surface of the monolith1 and the inner surface of the metal casing 2. Further, when mounted tothe catalytic converter, the holder has an initial bulk density of 0.18to 0.8 g/cm³, preferably 0.2 to 0.6 g/cm³, an initial surface pressureof 0 to 8 kg/cm². When the organic binder is thermally decomposed, therestoration surface pressure of the holder 3 mounted in the clearance(D) is in the range of 0.5 to 8 kg/cm², which amounts to 50 to 90% or110 to 150% of the initial surface pressure.

The afore-mentioned catalytic converter can be basically producedaccording to the below-mentioned process for the production of thecatalytic converter according to the seventh aspect of the presentinvention.

Next, the catalytic converter according to the sixth aspect of thepresent invention is described by referring to FIGS. 1 to 3.

The afore-mentioned catalytic converter comprises a metal monolith 1 ofa cylindrical shape for supporting a catalyst for cleaning exhaust gasesthereon, a metal casing 2 accommodating the monolith and connected toexhaust pipes, and a holder 3 inserted into a clearance (D) between anouter surface of the monolith 1 and an inner surface of the metal casing2. The monolith 1 which may be made of a ferrite-based stainless steelfoil and have a honeycomb structure, is supported directly by the holder3 produced by uniformly dispersing the organic binder capable of beingdissipated by thermal decomposition in the base mat compressed in itsthickness direction. When the afore-mentioned holder 3 is kept in such acompressed condition that the thickness of the holder corresponds to theclearance (D) between the outer peripheral surface of the monolith 1 andthe inner peripheral surface of the metal casing 2 and when the organicbinder is thermally decomposed, the holder exhibits a restorationsurface pressure of 0.1 to 4.0 kg/cm².

The catalytic converter of the afore-mentioned arrangement, whichcomprises the metal monolith 1 of a cylindrical shape for supporting acatalyst for cleaning exhaust gases thereon, the metal casing 2accommodating the monolith and connected to exhaust pipes and the holder3 inserted into a clearance (D) between an outer surface of the monolith1 and an inner surface of the metal casing 2, has substantially the samearrangement as that according to the fourth aspect of the presentinvention except for such limitations that the monolith 1 is made of thespecific metal and has the honeycomb structure.

One of the features of the present invention resides in the use of thespecific holder 3. That is, as described above, the crystalline aluminafiber mat (base mat) used in the catalytic converter according to thepresent invention has been disclosed in Japanese Patent ApplicationLaid-open (Kokai) No. 7-127443 (1995) in which the crystalline aluminafiber mat is used as a holder for a ceramic honeycomb catalyst(monolith). In this case, since the ceramic monolith exhibits a low heatexpansion as compared to the metal casing and the metal casing has ahigh coefficient of thermal expansion, a clearance therebetween becomeslarge, so that the holder composed of the crystalline alumina fiber matcannot provide sufficient supporting effects.

On the other hand, even if the ceramic monolith is simply replaced withthe monolith composed especially of the ferrite-based stainless steelfoil, the clearance between the monolith and the metal casing becomesconsiderably small so that the monolith is caused to be buckled. On thecontrary, in accordance with the present invention, as a material of theholder 3, there can be used the specific crystalline alumina mat havinga restoration surface pressure as low as 0.1 to 0.4 kg/cm² whencompressed so as to reduce the thickness to that corresponding to theclearance, whereby the holder can successfully exhibit sufficientsupporting effects without the buckling of the monolith 1.

Another feature of the present invention resides in that the monolith 1is supported directly by the holder 3 composed of a crystalline aluminamat. That is, in the case where such an arrangement in which a metalring having a short axial length is fitted on an outer peripheralsurface of a metal monolith, is adopted like the cleaning apparatus asdisclosed in Japanese Patent Application Laid-open (Kokai) No. 6-126191(1994), the buckling of the metal monolith is likely to occur due to atightening force temporarily exerted by the metal ring onto the metalmonolith having a large coefficient of thermal expansion. On the otherhand, in accordance with the present invention, such a structure inwhich the monolith 1 is supported directly by the holder 3 composed ofthe crystalline alumina fiber mat, is adopted so as not to limit thethermal expansion of the metal monolith.

The structure of the metal casing 2 in the afore-mentioned catalyticconverter is substantially the same as that of the catalytic converteraccording to the fourth aspect of the present invention.

The monolith 1 may be made of a metal foil material. Such a metal foilmaterial may be selected from those exhibiting low deterioration whensubjected to a high-temperature heat cycle such as high-temperatureoxidation, low thermal expansion and the like, a good compatibility withthe catalyst and a coating material used to carry the catalyst thereon,and low thermal degradation after the catalyst is carried thereon. Ingeneral, ferrite-based stainless steel foils composed substantially ofFe, Cr and Al or Si and having a low coefficient of thermal expansionmay be suitably used as the metal foil material.

The content of Cr contained in the stainless steel foil is generallyabout 20 wt %. Although a small content of Al is preferred, Al may begenerally added in an amount of about 5 wt % in view of its oxidationresistance. In addition, a small amount of other metals such as La, Ce,Y, Ti or the like may be also added to the stainless steel foil. Thethickness of the stainless steel foil is preferably as small as possibleto attain a low pressure loss or a good temperature-rise characteristic.The thickness of the stainless steel foil is generally not more than 100μm which is thinner than the thickness of ceramic monolith, butpreferably not less than 20 μm in view of its oxidation resistance.

The monolith 1 may have a laminated structure comprising a flat sheetand a corrugated sheet both made of the afore-mentioned stainless steelfoil. Specifically, the monolith may be formed into a honeycombstructure produced by alternately laminating the flat sheets and thecorrugated sheets, bonding these sheets with each other so as towithstand a high-temperature heat cycle and rolling the laminatedmaterial into a cylindrical shape, or by alternately laminating the flatsheets and the corrugated sheets, and winding and bonding these sheetsto form a cylindrical material. Any bonding methods such as brazing,diffusion-welding or the like can be used for the production of themonolith 1. The thus-prepared monolith 1 is coated with an alumina coatlayer and then carries thereon a noble metal layer such as Pt, Ph or thelike, thereby imparting a function as a catalyst thereto.

In the afore-mentioned catalytic converter, the holder 3 may be a moldedproduct composed of the non-expansive compressed mat and the organicbinder uniformly impregnated into the compressed mat and capable ofbeing dissipated by thermal decomposition. It is important that thecompressed mat has a restoration surface pressure of 0.1 to 4.0 kg/cm²,preferably 0.1 to 2.5 kg/cm², when the opposite surfaces thereof is insuch a compressed or fixed condition that it has a thicknesscorresponding to the clearance (D). The restoration surface pressure canbe exhibited only after the organic binder uniformly dispersed in thecompressed mat is dissipated by thermal decomposition thereof, so thatthe molded product can follow changes in size of the clearance (D)caused by the changes in temperatures of the monolith 1 and the metalcasing 2, whereby the monolith 1 can be elastically supported directlyby the molded product.

The crystalline alumina fiber mat is excellent in heat resistance ascompared with non-crystalline ceramic fiber made of the samealumina/silica-based material and limits a heat deterioration such assoftening or shrinkage to an extremely low level, similarly to theceramic fiber. Consequently, if the crystalline alumina fiber is used,there can be obtained a compressed mat having an excellent elasticity.That is, such a compressed mat can exhibit excellent properties such asa low bulk density, a high capability for retaining the catalyst and alow temperature change. Accordingly, even when the clearance (D) isnarrowed so that the bulk density of the compressed mat is rapidlyincreased, it is prevented to rapidly increase the retaining pressureexerted on the monolith 1. This indicates that the crystalline aluminafiber is an optimum material of the holder for the metal monolith.

Besides, since the afore-mentioned compressed mat shows a lowtemperature-dependency of the capability for retaining the catalyst,other kinds of holders such as metal rings, metal nets or the like asused in conventional apparatuses are not necessary to effectivelysupport the monolith without buckling thereof. Further, owing to theafore-mentioned inherent properties of the fiber itself, the compressedmat has such a feature that fly-off of the fiber is limited to a lowlevel when exposed to high-temperature exhaust gases. Accordingly, fromsuch a viewpoint, the crystalline alumina fiber can provide an excellentfiber applicable to the compressed mat for the holder 3.

In the afore-mentioned catalytic converter, the thickness (B) of thecompressed mat may be determined depending upon its elasticity, theclearance (D), its thermally-changing amounts, its gas-sealingproperties and a buckling strength of the monolith 1. In this case, itis required that the thickness of the compressed mat is selectivelydetermined so as to show a compressing force of 0.1 to 4.0 kg/cm², whencompressed so as to reduce the thickness to that corresponding to theclearance (D). In general, the molded product composed of the compressedmat having a ratio of its thickness to the ordinary-state thickness (A)of the base mat of not more than 1/1.25, preferably in the range of 1/2to 1/8, and the organic binder can satisfy the requirement of theafore-mentioned compressing force after the organic binder is dissipatedby thermal decomposition.

The holder used in the catalytic converter according to the sixth aspectof the present invention can be produced by the process according to thethird aspect of the present invention. The catalytic converter can besubstantially produced according to the below-mentioned process for theproduction of the catalytic converter according to the seventh aspect ofthe present invention.

In the afore-mentioned catalytic converter, the thickness (C) of theholder 3 (molded product) before mounting thereto is preferably 1.0 to1.7 times the clearance (D). The bulk density of the holder 3 ispreferably as low as possible from the standpoint of preventing thebuckling of the monolith 1 at a high temperature. However, when the bulkdensity is too low, the holder shows deteriorated capability forretaining the catalyst thereon. Accordingly, the bulk density of theholder 3 should be adjusted to an appropriate value. Specifically, it ispreferred that the bulk density of the holder 3 before mounting to thecatalytic converter be not more than 0.45 g/cm³ in the case where thefoil thickness of the monolith 1 is in the range of 60 to 100 μm, andnot more than 0.2 g/cm³ in the case where the foil thickness of themonolith 1 is in the range of 20 to 50 μm. The lower limit of such abulk density is preferably adjusted to not less than 0.06 g/cm³ so asnot to cause removal of the monolith 1 when subjected tohigh-temperature heat cycle experiments. Further, the bulk density ofthe holder when mounted to the clearance (D) is preferably in the rangeof 0.10 to 0.45 g/cm³.

In the afore-mentioned catalytic converter, the initial surface pressureof the holder 3 when mounted thereto is preferably in the range of 0 to0.45 kg/cm³. It is especially preferred that the restoration surfacepressure of the holder 3 received in the clearance (D) upon the thermaldecomposition of the organic binder may be in the range of 0.1 to 4kg/cm², and may range from 50 to 90% of the initial surface pressure ofthe holder mounted to the catalytic converter. Under such conditions,the clearance is narrowed and the surface pressure of the holder istemporarily increased. Once the organic binder is thermally decomposed,the surface pressure is decreased so that the monolith is effectivelyprevented from being buckled. When the organic binder is thermallydecomposed, the restoration surface pressure of the holder 3 may be inthe range of 0.1 to 4 kg/cm², and may range from 110 to 150% of theinitial surface pressure of the holder mounted to the catalyticconverter. The thickness-restoring ratio of the holder 3 is preferablyin the range of 1.25 to 8. It is preferred that the holder 3 has atensile strength of 1 to 30 kg/cm², and a tensile modulus of 150 to 600kg/cm².

In the afore-mentioned catalytic converter, the holder 3 can be producedby the process according to the third aspect of the present invention.In this case, the crystalline alumina fiber mat used in the first stepof the process has a ordinary-state thickness (A) which is preferably 2to 8 times the thickness (C) of the holder. The compressed thickness (B)of the compressed alumina fiber mat used in the second step of theprocess is preferably larger than the clearance (D) of the catalyticconverter. When the compressed thickness (B) is smaller than theclearance (D), there is a likelihood that the mat cannot exhibit asufficient restoring force due to the compression hysteresis. Further,the thickness (C) of the holder 3 obtained in the third step is 1 to 1.5times the compressed thickness (B) of the second step, so that theholder can exhibit the thickness-restoring property upon the thermaldecomposition of the organic binder when the opposite surfaces thereofare kept in an open state.

Next, the process for producing a catalytic converter, according to theseventh aspect of the present invention, is described below by referringto FIGS. 1 to 3.

The afore-mentioned process is adapted to produce the catalyticconverter comprising a monolith 1 of a cylindrical shape for supportinga catalyst for cleaning exhaust gases thereon, a metal casing 2connected to exhaust pipes and a holder 3 inserted into a clearance (D)between an outer surface of the monolith 1 and an inner surface of themetal casing 2, and comprises (I) the first step of winding the holder 3around an outer peripheral surface of the cylindrical monolith 1, and(II) the second step of accommodating the monolith 1 wound with theholder 3, in the metal casing having a two-piece structure composed ofan upper shell member 2a and a lower shell member 2b and then bringingthe upper and lower shell members 2a and 2b into mating contact witheach other, and connecting them together by welding mating peripheralportions thereof. The holder 3 comprises the base mat compressed in itsthickness direction and the organic binder uniformly dispersed in thebase mat and capable of being dissipated when subjected to thermaldecomposition. Further, in the case where the holder 3 is kept in such acompressed condition that the thickness thereof corresponds to theclearance (D) and the organic binder contained therein is subjected tothermal decomposition, the holder 3 can exhibit a restoration surfacepressure of 0.1 to 4.0 kg/cm².

In the afore-mentioned first step, the holder 3 is wound around theouter peripheral surface of the cylindrical monolith 1. The holder 3 isformed at opposite winding ends thereof with a recess and a projectionwhich are brought into mating engagement with each other aftercompletion of the winding.

In the afore-mentioned second step, the monolith 1 around which theholder is completely wound is held in place within the lower shellmember 2b and then the upper shell member 2a is brought into matingcontact with the lower shell member 2b. The upper and lower shellmembers 2a and 2b are connected at their peripheral mating portions toeach other by welding. In this case, if the thickness (C) of the holder3 is generally not more than 2 times, preferably not more than 1.7times, more preferably 1.6 times the clearance (D), a part of the holdercan be prevented from projecting outwardly and being interposed betweenflange portions 21a and 21b.

The catalytic converter illustrated hereinbefore comprises the two-piecemetal casing 2. Alternatively, if the metal casing 2 is of an integralcylindrical shape, the monolith 1 around which the holder 3 is wound canbe mounted within the metal casing only by pushing the monolith throughan open end of the metal casing. In this case, the thickness (C) of theholder 3 is preferably about 1.0 to about 1.7 times the clearance (D).

Next, the process for producing a catalytic converter, according to theeighth aspect of the present invention, is described below by referringto FIGS. 1 to 3.

The afore-mentioned process is adapted to produce the catalyticconverter comprising a metal monolith 1 of a cylindrical shape forsupporting a catalyst for cleaning exhaust gases thereon, a metal casing2 accommodating the monolith 1 therein and connected to exhaust pipesand a holder 3 inserted into a clearance (D) between an outer surface ofthe monolith 1 and an inner surface of the metal casing 2, wherein themonolith 1 which has a honeycomb structure and is composed offerrite-based stainless steel foils, is supported directly by the holder3, and the holder 3 is produced by the process comprising (I) the firststep of impregnating a base mat with an organic binder solution, (II)the second step of compressing the base mat impregnated with the organicbinder solution in the thickness direction and (III) the third step ofremoving a solvent of the organic binder solution while maintaining thethickness of the compressed base mat. Further, in the case where theholder 3 is kept in such a compressed condition that the thicknessthereof corresponds to the clearance (D) and the organic bindercontained therein is subjected to thermal decomposition, the holder 3can exhibit a restoration surface pressure of 0.1 to 4.0 kg/cm². In theprocess according to the eighth aspect of the present invention, thesame elements as used in the catalytic converter according to the sixthaspect of the present invention can be also used.

As will be understood from the foregoing, the present invention canprovide the below-mentioned excellent effects.

That is, the holder according to the present invention and produced bythe process according to the present invention is composed of acompressed alumina fiber body having an excellent durability, therebypreventing corrosion or degradation thereof even when exposed to exhaustgases emitted from an internal combustion engine and having a high flowrate and a high temperature. Besides, since the organic binder containedin the holder can be thermally decomposed and dissipated when exposed tothe high-temperature exhaust gases, the holder restores its thicknessand exhibits an elasticity, so that the clearance between the monolithand the casing can be completely closed to prevent a leakage of theexhaust gases and fly-off of the fiber when exposed to the exhaustgasses having a high flow rate. As a result, the monolith can beretained by a predetermined retention force within the metal casing in astable manner.

The afore-mentioned holder can also follow change with time in size ofthe clearance due to change in temperature of the monolith and the metalcasing, so that the monolith can be elastically secured within the metalcasing. Further, the holder has a thickness-restoring property by whichthe holder can restore its thickness to an initial thickness of thealumina fiber mat as a base mat, so that the monolith can be stablyretained within the casing for a long period of time.

Also, when the afore-mentioned holder is assembled into the catalyticconverter, the thickness of the holder is kept much thinner by a bondingforce of the organic binder so that the holder can be readily mountedinto an interior of the catalytic converter without damage to the holderitself.

The catalytic converter according to the present invention and producedby the process according to the present invention also uses theafore-specified holder. Accordingly, the catalytic converter canwithstand violent vibration or impact for a long period of time.Besides, since the holder is readily mounted to the catalytic converter,the production of the catalytic converter is highly facilitated, therebyreducing the production cost of the catalytic converter.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention is described in more detail by way ofexamples. However, the present invention is not limited to theseexamples but intended to involve any variations, changes ormodifications thereof, unless they departs from the scope or sprits ofthe appended claims. Incidentally, in the following descriptions, thereference numerals used in FIGS. 1 to 3 are also cited and the valuesfrom the Examples and Comparative Examples are enumerated in Tables 1 to3. In addition, "%" represents "% by weight" unless otherwise specified.

EXAMPLE 1

As a base mat, a mullite fiber mat having a ordinary-state thickness (A)of 16 mm, a width of 600 mm and a length of 3600 mm (tradename:"MAFTECBLANKET" produced by Mitsubishi Chemical Corporation, having abulk density of 0.10 g/cm³, an average fiber diameter of 4 μm and afiber length of 20 to 200 mm and containing 72% of alumina component and28% of silica component) was prepared.

When the base mat was compressed so as to reduce its thickness to 5.5mm, the pressure of 2.0 kg/cm² was required to maintain the compressedthickness (B) of the compressed mat. The compressed mat was maintainedat the compressed thickness (B) for 5 minutes and then the pressure wasreleased, so that the mat was found to have a restoring property capableof restoring its thickness up to 16 mm.

Next, the afore-mentioned base mat was immersed for 1 minutes in a raworganic binder solution (tradename: "LX874" produced by Nippon Zeon Co.,Ltd., an aqueous dispersion-type emulsion of acrylate rubbers having asolid content of 45%). Successively, the base mat immersed in theorganic binder solution was pressingly sandwiched at the oppositesurfaces thereof between two polyester nets each having a mesh size of0.33 mm and further at the opposite outsides thereof between twopunching metals (having a hole diameter of 3.5 mm, an opening percentageof 22.7% and a thickness of 2.3 mm). Thereafter, the base mat waspressed while evacuating on one surface side thereof so as to becompressed to reduce its thickness up to the compressed thickness (B) of5.5 mm. Next, while maintaining the compressed thickness (B), thecompressed base mat is dried by a hot air at 100° C. for 3 hours. Aftercooling, the polyester nets and the punching metals were removed fromthe compressed base mat to obtain the aimed holder 3.

The thus-obtained holder 3 had a thickness (C) of 6.0 mm and the (solid)content of the organic binder was 13 parts by weight based on 100 partsby weight of the alumina fiber mat. It was ascertained that the pressurerequired to compress the holder 3 to the thickness of 4 mm correspondingto the clearance (D) was 4.1 kg/cm², and the surface pressure, namelythe initial surface pressure, of the holder was 4.1 kg/cm² when mountedto the catalytic converter having the designed clearance (D) of 4 mm. Inaddition, the thus-obtained holder 3 was cut into strips each having awidth of 20 mm and a length of 150 mm. The strips having a width of 20mm and a span of 80 mm were tested by a tensile testing machine(tradename "Shimadzu Autograph IS-500" manufactured by ShimadzuSeisakusho Co., Ltd.) to measure tensile strength and tensile modulusthereof at stress rate of 5 mm/min (refer to Tables).

Incidentally, if a test specimen has a length of l₀, an elongation ofΔl, a tensile strength of Pn and a cross-sectional area of A, a tensilemodulus E of the test specimen is given by the following formula:

    E=(Pn/A)/(66 l/l.sub.0)

    Unit tensile strength: Pa=Pn/A

Next, the thus-obtained holder 3 was cut into a test specimen. The testspecimen was placed in an oven maintained at 800° C. and heated for onehour to thermally decompose the organic binder contained therein. As aresult, the thickness (C) of the holder was restored to 15 mm. It wasconfirmed that the thickness-restoring ratio of the holder was 2.5 whencalculated according to the above formula (I).

Further, after the thickness of the holder 3 was restored, the holderwas compressed so as to reduce its thickness to 4 mm corresponding tothe clearance (D). The pressure required to maintain the thickness ofthe holder corresponding to the clearance (D) was 4.0 kg/cm².Accordingly, it was confirmed that after the thus-obtained holder 3 wasmounted to the catalytic converter designed to have the clearance (D) of4 mm and the organic binder was burned, the holder was able to be fixedtherein with a restoration surface pressure of 4.0 kg/cm².

Next, the thus-obtained holder 3 (having the thickness (C) of 6.0 mm)was cut into a strip having a width of 80 mm and a length of 320 mm.After the strip was wound around an outer peripheral surface of themonolith 1 as shown in FIG. 2, the monolith 1 was held in place withinthe two-piece metal casing 2 (clearance: 4 mm) having such a structureas shown in FIG. 1, to examine a canning property thereof. As a result,the cut strip could be readily wound around the outer peripheral surfaceof the monolith 1 in a close contact manner. Further, when mounted intoan interior of the metal casing 2, the strip was prevented fromprojecting outwardly and being sandwiched between the flange portions21a and 21b. It was confirmed that the strip exhibited an excellentcanning property.

EXAMPLES 2 to 7

The same procedure as defined in Example 1 was conducted except thatbase mats each having a bulk density and an ordinary-state thickness (A)as shown in Tables were compressed so as to reduce its thickness to acompressed thickness (B) as also shown in Tables, so that holders 3having a thickness (C) were obtained.

Each of the thus-obtained holders 3 had a (solid) content of the organicbinder as shown in Tables. The pressure required to compress thethus-obtained holder 3 so as to reduce its thickness to thatcorresponding to the clearance (D), namely the initial pressure, and aunit tensile strength and a tensile modulus thereof measured in the samemanner as in Example 1 are shown in Tables.

Further, the thus-obtained holder 3 was cut into a test specimen whichwas then subjected to the thermal decomposition of the organic bindercontained therein in the same manner as described in Example 1. Thethickness-restoring ratio and the pressure required to compress theholder 3 having a restored thickness so as to reduce the thickness tothat corresponding to the clearance (D), namely the surface pressureafter burning, are also shown in Tables.

Next, in order to examine a canning property of each of the catalyticconverters, the same procedures as defined in Example 1 were conductedexcept that two-piece metal casings 2 having the clearances (D) as shownin Tables were used. As a result, the cut test specimen could be readilywound around the outer peripheral surface of the monolith 1 in a closecontact manner, and that when the monolith was inserted into an interiorof the metal casing, the holder was prevented from projecting outwardlyand being sandwiched between the flange portions 21a and 21b.Accordingly, it was confirmed that the catalytic converters had anexcellent canning property.

COMPARATIVE EXAMPLE 1

In the same manner as described in Example 1, an alumina fiber mat wasimpregnated with an organic binder solution and then pressinglysandwiched at the opposite surfaces thereof between two polyester netseach having a mesh size of 0.33 mm and further at the opposite outsidesthereof between two punching metals (having a hole diameter of 3.5 mm,an opening rate of 22.7% and a thickness of 2.3 mm). Thereafter, thealumina fiber mat was pressed while evacuating at one surface thereofand compressed so as to reduce its thickness to the compressed thickness(B) of 5.5 mm. Next, instead of the procedure as described in Example 1in which the mat was dried while maintaining the compressed thickness(B), the one polyester net and the one punching metal both placed on thesame side of the mat were removed, and then the alumina fiber mat fromone side of which the polyester net and punching metal was removed wasdried, so that a molded product having a thickness (C) of 13 mm andcontaining 13 parts by weight of the organic binder (solid content)could be obtained (see Tables).

Next, the thus-obtained molded product (having the thickness (C) of 13mm) was cut into a strip as a holder having a width of 80 mm and alength of 320 mm. It was attempted to mount the holder together with themonolith to the two-piece metal casing having such a structure as shownin FIG. 1. However, it was difficult to wind the bulky strip-shapedholder around the outer peripheral surface of the monolith 1. Further,at the time of canning, the holder considerably projected outwardly andtherefore a large portion of the holder was sandwiched between theflange portions 21a and 21b. As a result, the holder could not bemounted within the metal casing (see Tables).

EXAMPLE 8

As a base mat, a mullite fiber mat having a ordinary-state thickness (A)of 12 mm, a width of 600 mm and a length of 3600 mm (tradename:"MAFTECBLANKET" produced by Mitsubishi Chemical Corporation, having abulk density of 0.1 g/cm³, an average fiber diameter of 4 μm and a fiberlength of 20 to 200 mm and containing 72% of alumina component and 28%of silica component) was prepared.

Next, the base mat was treated in the same manner as defined in Example1 to obtain a holder 3 having a thickness (C) of 6.0 mm and a bulkdensity of 0.23 g/cm³ and containing 15 parts by weight of the organicbinder (solid content). When the afore-mentioned base mat having noorganic binder was compressed so as to reduce its thickness up to 4 mm,the pressure required was 2.1 kg/cm². When the molded productimpregnated with the organic binder was also compressed so as to reduceits thickness up to 4 mm, the pressure required, namely the initialsurface pressure was 2.2 kg/cm². As a result, it was confirmed that theorganic binder could restrict the restoration of the thickness of thecompressed mat without damages to properties thereof. In addition, thethus-obtained holder 3 was measured for its thickness-restoring ratio bythe same method as defined in Example 1. The measurement revealed thatthe thickness-restoring ratio was 2.0 (see Tables).

Next, a stainless steel foil in the form of a flat foil comprising 20%of Cr and 5% of Al, and having a thickness of 60 μm, and a corrugatedfoil having a wave height of 1.25 mm and a pitch of 2.5 mm werelaminated together and then rolled. The rolled foils were joinedtogether by diffusion method in a vacuum furnace to prepare a monolith 1having a diameter of 89 mm and a length of 130 mm. Successively, afteran alumina coating layer was formed on the thus-prepared monolith 1, Ptand Ph as catalytic components were supported thereon. Theafore-mentioned holder 3 was wound around the outer peripheral surfaceof the monolith 1 on which the catalyst was supported, in such a manneras illustrated in FIG. 2. The monolith 1 was accommodated within themetal casing 2 having such a structure as illustrated in FIG. 1 toproduce a catalytic converter. When calculated from a value of theclearance (D), the thickness (C) and the bulk density upon mounting ofthe holder 3 were 4.0 mm and 0.35 g/cm³, respectively (see Tables).

In the afore-mentioned assembling, it was ascertained that the holder 3was appropriately mounted to the metal casing without leaking a part ofthe holder to be sandwiched between the flange portions 21a and 21b.Successively, the thus-assembled catalytic converter was mounted to anengine-testing apparatus and subjected to a high-temperature heat cycletest at a temperature from a normal temperature to 830° C. for 600cycles. As a result, it was found that any leakage from the outerperipheral surface of the monolith 1 due to poor sealing was not caused.Further, the disassembling of the catalytic converter after the testrevealed that any displacement and buckling of the monolith 1 were notcaused.

EXAMPLE 9

The same procedure as defined in Example 8 was conducted except that abase mat having an ordinary-state thickness (A) of 15 mm was used, sothat a holder 3 having a thickness (C) of 6.8 mm and a bulk density of0.26 g/cm³, and containing 20 parts by weight of the organic binder(solid content) was obtained.

When the thus-obtained holder 3 was compressed so as to reduce itsthickness up to 4.0 mm, it was confirmed that the pressure requiredtherefor, namely the initial pressure, was 4.0 kg/cm². Thethickness-restoring ratio and the surface pressure after burning bothmeasured in the same manner as defined in Example 1 were shown inTables.

Next, the same procedure as defined in Example 8 was conducted toproduce a catalytic converter. The thickness (C) of the holder 3 mountedwithin the metal casing 2 and the bulk density of the holder 3 uponmounting were 4.0 mm and 0.45 g/cm³, respectively (see Tables).

Successively, the thus-produced catalytic converter was subjected to ahigh-temperature heat cycle test in the same manner as defined inExample 8. As a result, it was found that any leakage from the outerperipheral surface of the monolith 1 due to poor sealing was not caused.However, since slight deformation of the monolith 1 was observed, theabove-measured bulk density of the holder upon mounting was consideredto be an upper limit thereof.

EXAMPLE 10

A base mat having the same composition as used in Example 1 except foran ordinary-state thickness (A) of 8 mm and a bulk density of 0.05g/cm³, was impregnated with the same organic binder as used in Example1, so that a molded product as a holder 3 having a thickness (C) of 6.0mm and a bulk density of 0.07 g/cm³ and containing 5 parts by weight ofthe organic binder (solid content) was obtained. When the thus-obtainedholder 3 was compressed so as to reduce its thickness up to 4.0 mm, itwas confirmed that the pressure required therefor was 0.1 kg/cm².Successively, a monolith 1 carrying a catalyst thereon was producedunder the same conditions as defined in Example 1 and accommodatedwithin a metal casing 2 to produce a catalytic converter. The holder 3mounted within the metal casing 2 had a thickness (C) of 4.0 mm and abulk density upon mounting of 0.105 g/cm³ (see Tables).

Successively, the thus-produced catalytic converter was subjected to ahigh-temperature heat cycle test in the same manner as defined inExample 1. As a result, it was found that any buckling of the monolith 1and any leakage from the outer peripheral surface of the monolith 1 dueto poor sealing were not caused. However, since slight displacement ofthe monolith 1 was observed, the above-measured bulk density of theholder upon mounting was considered to be a lower limit thereof.

EXAMPLE 11

The same procedure as defined in Example 8 was conducted except that abase mat having an ordinary-state thickness (A) of 6 mm was used, sothat a holder 3 having a thickness (C) of 5.5 mm and a bulk density of0.13 g/cm³, and containing 20 parts by weight of the organic binder(solid content) was obtained.

When the thus-obtained holder 3 was compressed so as to reduce itsthickness up to 4.0 mm, it was confirmed that the pressure requiredtherefor, namely the initial pressure, was 0.6 kg/cm². In addition, theunit tensile strength, the tensile modulus, the thickness-restoringratio and the surface pressure after burning all measured in the samemanner as defined in Example 1 were shown in Table.

Next, a stainless steel foil in the form of a flat foil comprising 13%of Cr and 2% of Si and having a thickness of 40 μm, and a corrugatedfoil having a wave height of 1.25 mm and a pitch of 2.5 mm werelaminated together and then rolled. The rolled foils were joinedtogether by diffusion method in a vacuum furnace to prepare a monolith 1having a diameter of 89 mm and a length of 130 mm. Successively, thecatalyst was supported on the thus-prepared monolith in the sameconditions as defined in Example 1. The afore-mentioned holder 3 waswound around the outer peripheral surface of the monolith 1 on which thecatalyst was supported, in such a manner as illustrated in FIG. 2. Themonolith 1 was accommodated within the metal casing 2 having an innerdiameter of 97 mm to produce a catalytic converter. The thickness (C)and the bulk density upon mounting of the holder 3 were 4.0 mm and 0.18g/cm³, respectively (see Tables).

The thus-produced catalytic converter was subjected to ahigh-temperature heat cycle test in the same manner as defined inExample 1. As a result, it was found that any displacement of themonolith 1, any leakage from the outer peripheral surface of themonolith 1 due to poor sealing and any buckling of the monolith 1 werenot caused. Further, the analysis after the test revealed that noleakage nor buckling of the monolith 1 were caused.

COMPARATIVE EXAMPLE 2

The same procedure as defined in Example 8 was conducted except that acommercially-available thermally-expansive base mat having anordinary-state thickness (A) of 5.5 mm and a bulk density of 0.61 g/cm³was used, so that a catalytic converter was produced. The thus-producedcatalytic converter was subjected to a high-temperature heat cycle testin the same manner as defined above. As a result, when 50 cycles wasreached, the monolith 1 was slipped and removed from the holder. It wasrevealed that buckling was caused over the entire outer peripheralsurface of the monolith 1 thus removed, and that the diameter of themonolith was reduced from 89 mm upon mounting to 86 mm.

                  TABLE 1                                                         ______________________________________                                                                 Amount                                               Base mat        Com-     of      Holder                                               Thick-  Bulk    pressed                                                                              binder                                                                              Thick-                                                                              Bulk                               Example ness    density thickness                                                                            (part by                                                                            ness  density                            No.     A (mm)  (g/cm.sup.3)                                                                          B (mm) weight)                                                                             C (mm)                                                                              (g/cm.sup.3)                       ______________________________________                                        Example 1                                                                             16      0.1     5.5    13    6.0   0.30                               Example 2                                                                             16      0.05    5.5    20    6.0   0.16                               Example 3                                                                             12      0.05    5.5    20    5.5   0.13                               Example 4                                                                             35      0.05    3.0    17    3.5   0.60                               Example 5                                                                             16      0.1     6.0    25    7.0   0.29                               Example 6                                                                             16      0.1     6.0    25    7.0   0.29                               Example 7                                                                             16      0.1     6.0    25    7.0   0.29                               Example 8                                                                             12      0.1     5.5    15    6.0   0.23                               Example 9                                                                             15      0.1     5.5    20    6.8   0.26                               Example 10                                                                            8       0.05    5.5    5     6.0   0.07                               Example 11                                                                            6       0.1     5.5    20    5.5   0.13                               Comparative                                                                           16      0.1     5.5    13    13    0.14                               Example 1                                                                     Comparative                                                                           5.5     0.61    5.5    15    6.0   --                                 Example 2                                                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                         Restoration                                                   Initial  Bulk   surface Bulk                                                  pressure density                                                                              pressure                                                                              density                                               upon     upon   after   after                                Example Clearance                                                                              mounting mounting                                                                             burning burning                              No.     D (mm)   (kg/cm.sup.2)                                                                          (g/cm.sup.3)                                                                         (kg/cm.sup.2)                                                                         (g/cm.sup.3)                         ______________________________________                                        Example 1                                                                             4.0      4.1      0.45   4.0     0.40                                 Example 2                                                                             4.0      0.5      0.24   0.9     0.20                                 Example 3                                                                             4.0      0.6      0.18   0.6     0.15                                 Example 4                                                                             3.0      6.9      0.70   9.8     0.58                                 Example 5                                                                             4.0      5.0      0.50   4.3     0.40                                 Example 6                                                                             7.0      0        0.29   1.1     0.23                                 Example 7                                                                             6.5      0.3      0.31   1.5     0.25                                 Example 8                                                                             4.0      2.2      0.35   2.1     0.30                                 Example 9                                                                             4.0      4.0      0.45   3.8     0.38                                 Example 10                                                                            4.0      0.1      0.11   0.1     0.10                                 Example 11                                                                            4.0      0.6      0.18   0.6     0.15                                 Comparative                                                                           4.0      Difficult to install                                         Example 1                                                                     Comparative                                                                           4.0      Removal or buckling of monolith                              Example 2                                                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                Ratio         Unit                                                            between thickness                                                                           tensile  Tensile                                                                              Thickness-                              Example of holder     strength modulus                                                                              restoring                               No.     C/B    A/C     C/D  ((kg/cm.sup.2)                                                                       (kg/cm.sup.2)                                                                        ratio                               ______________________________________                                        Example 1                                                                             1.1    2.7     1.5  24     550    2.5                                 Example 2                                                                             1.1    2.7     1.5  23     580    2.5                                 Example 3                                                                             1.0    2.2     1.4  19     390    2.2                                 Example 4                                                                             1.2    10      1.2  25     500    8.6                                 Example 5                                                                             1.2    2.3     1.8  34     610    2.1                                 Example 6                                                                             1.2    2.3     1.0  34     610    2.1                                 Example 7                                                                             1.2    2.3     1.1  34     610    2.1                                 Example 8                                                                             1.1    2.0     1.5  28     540    2.0                                 Example 9                                                                             1.2    2.2     1.7  23     370    2.2                                 Example 10                                                                            1.1    1.3     1.5   3     170    1.3                                 Example 11                                                                            1.0    1.1     1.4  19     360    1.1                                 Comparative                                                                           2.4    1.2     3.3  --     --     --                                  Example 1                                                                     Comparative                                                                           --     --      --   --     --     --                                  Example 2                                                                     ______________________________________                                    

INDUSTRIAL APPLICABILITY

The holder for monolith according to the present invention is readilymounted within a casing while being maintained in an compressed state,and restores its thickness when an organic binder contained therein isthermally decomposed, so that the monolith can be stably supported inthe casing for a long period of time, and a gas sealing propertiesthereof can be assured. In addition, a catalytic converter using theafore-specified holder for monolith can stably retain the monolithagainst violent vibration or impact and can exhibit an excellentdurability. Accordingly, the present invention is effectively applicableto a high-performance catalytic converter mounted to internal combustionengines of various vehicles for which a high cleaning efficiency isrequired.

What is claimed is:
 1. A process for producing a monolith-holdingelement for use in a catalytic converter having a cylindrical monolithsupporting a catalyst for cleaning exhaust gases thereon, a metalcasing, accommodating said monolith therein, connected to exhaust pipes,said monolith-holding element fitted into a clearance between an outersurface of the monolith and an inner surface of the metal casing, theprocess comprising:providing an alumina fiber mat having a firstuncompressed thickness, the alumina fiber mat having a bulk density of0.05 to 0.20 g/cm³ ; impregnating the alumina fiber mat with a solutioncontaining a solvent and an organic binder capable of being dissipatedby thermal decomposition; compressing the alumina fiber mat impregnatedwith the organic binder-containing solution in the thickness directionso as to produce a second compressed thickness thereof reduced by 1/2 to1/15 times the first uncompressed thickness; removing the solvent whilesimultaneously maintaining the second compressed thickness of thealumina fiber mat, leaving the organic binder within the compressedalumina fiber mat; releasing the compression to provide amonolith-holding element having an uncompressed third thickness of 1 to1.5 times the second compressed thickness of the alumina fiber mat, theremaining organic binder having a binding force sufficient to suppressthe restoration of the alumina fiber mat to the first uncompressedthickness; fitting the monolith-holding element into the clearancebetween the outer surface of the monolith and the inner surface of themetal casing; and, thermally decomposing the organic binder such thatthe monolith-holding element exhibits a thickness restoring propertywith opposite surfaces thereof being kept in an open state, and arestoration surface pressure when kept under a compressed conditioncorresponding to the clearance, in the range of 0.5 to 30 kg/cm².
 2. Aprocess for producing a catalytic converter comprising: providing acylindrical monolith supporting a catalyst for cleaning exhaust gasesthereon, said monolith being composed of a ferrite-based stainless steelfoil and having a honeycomb structure;providing a metal casing connectedto exhaust pipes, providing an alumina fiber mat having a firstuncompressed thickness, impregnating the alumina fiber mat with asolution containing an organic binder, compressing said alumina fibermat impregnated with said organic binder-containing solution in thethickness direction to a second compressed thickness; simultaneouslyremoving a solvent of said organic binder-containing solution whilemaintaining the second thickness of the compressed alumina fiber mat,leaving the organic binder in the compressed alumina fiber mat:releasing the compression such that a monolith holding element isproduced having a third uncompressed thickness that is 1.0 to 1.5 timesthe second thickness, the remaining organic binder preventingrestoration of the alumina mat to the first uncompressed thickness;fitting the monolith-holding element having the third uncompressedthickness into a clearance between an outer peripheral surface of saidmonolith and an inner peripheral surface of said metal casing, whereinwhen said organic binder is thermally decomposed, a restoration surfacepressure of said monolith-holding element being kept under a compressedcondition where the thickness thereof is reduced to that correspondingto said clearance, is in the range of 0.1 to 4.0 kg/cm².